The cell surface protein designated elk is a member of a family of proteins known as the tyrosine kinase receptors. Proteins of this family have an intrinsic kinase activity that is activated upon ligand binding. A partial clone of elk was first discovered in a rat brain cDNA expression library that was screened for proteins expressing tyrosine kinase activity (Letwin et al., Oncogene 3:621, 1988). Later, a composite sequence spanning the entire elk coding region was derived from partial clones isolated from a rat brain cDNA library and a rat cerebellar brain library using the partial clone as a probe (Lhotak et al., Mol. Cell. Biol. 11:2496, 1991). The elk protein is very closely related to a number of other receptor tyrosine kinases, including hek (Boyd et al. J. Biol. Chem. 267:3262, 1992 and Wicks et al. Proc. Natl. Acad. Sci. USA 89:1611, 1992); the hek homologs mek4 and cek4 (Sajjadi et al. New Biol. 3:769, 1991); eek (Chan et al. Oncogene 6:1057, 1991); erk (Chan et al. supra.), eck (Lindberg et al. Mol. Cell. Biol. 10:6316, 1990); cek5 (Pasquale, E.B. Cell Regulation 2:523, 1991); and eph (Hirai et al. Science 238:1717, 1987). The proteins of this subfamily are related not only in their cytoplasmic domains, but also in their extracellular domains, which are 41 to 68% identical. Interestingly, the tissue distributions of these various receptors are diverse. For example, expression of elk mRNA has been shown to be limited to testis and brain (Lhotak et al., supra), whereas eck is found not only in these same two tissues but in lung, intestine, kidney, spleen, ovary, and skin as well.
Ligands for the receptor tyrosine kinases are a diverse group of proteins that affect the growth, differentiation, and survival of cells expressing the receptors. To date, no ligand for elk has been discovered. Identification of the putative ligand would prove useful in investigating the nature of cellular processes that might be regulated by the elk protein.